Method of making an induction motor rotor

ABSTRACT

A method for fabricating an induction motor rotor lamination includes using an indexed notching die to punch a plurality of closed rotor slots at different distances from a center point of rotation by punching a first rotor slot, changing the position of a center point of the rotor lamination with respect to the notching die, and punching a second rotor slot. In another embodiment, a plurality of rotor slit patterns are punched using a variable depth indexed notching die including at least two rotor slit punching portions, one being deeper than another, by punching a first rotor slit pattern, changing the depth of closure of the notching die, and punching a second rotor slit pattern with the second rotor slit pattern having a different number of rotor slits than the first. In another embodiment, each rotor slit is open to and off center with respect to its respective rotor slot, and rotor laminations are stacked by positioning some of the rotor laminations in an orientation with is turned over and rotated with respect to others of the rotor laminations; filling the rotor slots and the rotor slits with cage material; and machining outer surfaces of the rotor laminations. In another embodiment, uniformly spaced identical rotor slots are punched and modulation is created by punching a contoured outside diameter or by inserting slot wedges of molded magnetic material.

BACKGROUND OF THE INVENTION

Closed loop operation of induction motor drive systems without shafttransducers is commonly referred to as sensorless, tachless, ortransducerless. Eliminating the need for rotor shaft transducers such asencoders, resolvers, and tachometers for position and velocityestimations in an induction motor drive system can reduce system cost,improve system reliability, and reduce the total motor package size.

Several approaches for providing sensorless control of induction motorsare described in U.S. Pat. Nos. 5,565,752 and 5,585,709 wherein adeterminist spatial modulation in the rotor leakage inductance of theinduction motor is tracked via the demodulation of an injected AC signalvoltage or current. Tracking of the modulated rotor leakage inductanceprovides an estimate of the rotor position and velocity that can then beused for sensorless control.

U.S. Pat. No. 5,565,752 describes induction motor rotors with spatialvariations in the rotor leakage inductance to enable rotor position andvelocity tracking. The patent describes special rotor slot designs thathave either open slots or a combination of open and closed slots.Although large motors often have bar wound rotors with open slots, themajority of conventional small induction motors, however, are fabricatedwith closed rotor slots with the rotor bars being costeffectively formedby casting molten aluminum (these rotors are usually machined andsometimes the bridges are removed and the slots are opened aftercasting). U.S. Pat. No. 5,565,752 does not disclose manufacturingmethods or rotor designs that are readily manufacturable with existingindustrial casting processes and equipment.

SUMMARY OF THE INVENTION

It is therefore seen to be desirable to provide a cost effective methodof creating spatial modulation. The present invention provides rotorlamination fabrication techniques to create the desired spatialmodulation in the rotor leakage inductance and/or the rotor resistancewhich can be used with AC signal injection to track rotor positions andvelocities.

Briefly, in accordance with one embodiment of the present invention, amethod for fabricating an induction motor rotor lamination comprisesusing an indexed notching die to punch a plurality of closed rotor slotsat different distances from a center point of rotation by first punchinga first rotor slot, changing the position of a center point of the rotorlamination with respect to the notching die, and then punching a secondrotor slot.

Another method for fabricating an induction motor rotor laminationcomprises: punching a plurality of closed rotor slots and rotor slits ina lamination; and using an indexed notching die having a wider rotorslit to selectively punch and widen predetermined ones of the rotorslits.

Another method for fabricating an induction motor rotor laminationcomprises: punching a plurality of rotor slit patterns using a variabledepth indexed notching die including at least two rotor slit punchingportions, one of the rotor slit punching portions being deeper thananother of the rotor slit punching portions, by punching a first rotorslit pattern, changing the depth of closure of the notching die, andpunching a second rotor slit pattern, the second rotor slit patternhaving a different number of rotor slits than the first rotor slitpattern.

Another method for fabricating an induction motor rotor comprises:providing a plurality of rotor laminations each including a plurality ofrotor slots situated at different distances from a center point ofrotation and a plurality of rotor slits, each rotor slit being open toand off center with respect to its respective rotor slot; stacking theplurality of rotor laminations by positioning some of the rotorlaminations in an orientation which is turned over and rotated withrespect to others of the rotor laminations; filling the rotor slots andthe rotor slits with cage material; and machining outer surfaces of therotor laminations.

In another embodiment of the present invention, uniformly spacedidentical rotor slots are punched and modulation is created by punchinga contoured outside diameter or by inserting slot wedges of moldedmagnetic material.

BRIEF DESCRIPTION OF THE DRAWINGS

The features of the invention believed to be novel are set forth withparticularity in the appended claims. The invention itself, however,both as to organization and method of operation, together with furtherobjects and advantages thereof, may best be understood by reference tothe following description taken in conjunction with the accompanyingdrawings, where like numerals represent like components, in which:

FIG. 1 is a top view of a two-pole rotor lamination of the presentinvention.

FIG. 2 is a top view of rotor lamination positions during die punchingof the rotor lamination of the embodiment of FIG. 1.

FIG. 3 is top view of a partially fabricated rotor lamination of thepresent invention.

FIG. 4 is a top view of a masking lamination which can be used duringrotor cage casting to prevent molten material from entering the airslits in the rotor lamination of FIG. 3.

FIG. 5 is a top view of the lamination of FIG. 3 after casting andmachining to the desired rotor outside diameter.

FIG. 6 is a top view of another rotor lamination of the presentinvention.

FIG. 7 is a top view of a partially fabricated rotor lamination of thepresent invention.

FIG. 8 is a perspective view of a variable depth notching die of thepresent invention.

FIGS. 9 and 10 are views of rotor slots and air slits which can befabricated with the notching die of FIG. 8.

FIG. 11 is a top view of another rotor lamination of the presentinvention showing a two pole design formed by the die of FIG. 8.

FIG. 12 is a top view of another rotor lamination of the presentinvention.

FIGS. 13-16 are top views illustrating another fabrication technique forrotor laminations.

FIGS. 17 and 18 are sectional side views of selected lamination stackingembodiments.

FIG. 19 is a top view of two rotor slots of the present invention.

FIG. 20 is a top view of the rotor slots of FIG. 19 in a lamination foruse in a two pole machine.

FIG. 21 is a top view of rotor slots in a lamination for use in a fourpole machine.

FIGS. 22-23 are top views of additional rotor slot shape variations ofthe present invention.

FIG. 24 is a simulation graph of rotor bar resistances as a function ofrotor excitation for the embodiment of FIG. 23.

FIGS. 25-30 are top views of additional rotor slot shape variations ofthe present invention.

FIG. 31 is a simulation graph of rotor bar resistances as a function ofrotor excitation for the embodiment of FIG. 30.

FIG. 32 is a top view of an additional rotor slot shape variation of thepresent invention.

FIG. 33 is a top view of a rotor lamination of the present inventionhaving an oblong shape.

FIGS. 34-38 are top views of rotor laminations of the present inventionillustrating the use of magnetic inserts.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

Some of the rotor slot shape and rotor bar embodiments of the presentinvention relate to inductance modulation, whereas others of theembodiments of the present invention relate to resistance modulation.Typically on larger machines (such as machines with greater than tenhorsepower, for example), inductance modulation is most useful due tothe fact that a high percentage of the total impedance results frominductance. On smaller machines (such as machines with less than tenhorsepower, for example), inductance modulation, resistance modulation,or a combination of inductance modulation and resistance modulation canbe useful for sensorless motor control. Preferably, for inductancemodulation and particularly for resistance modulation, the crosssectional areas of each rotor slot shape are within one percent of eachother such that the DC resistances of each bar are essentiallyidentical. A spatial variation in the DC resistances will createundesirable torque ripple at the fundamental component slip frequency.

FIGS. 1-18 illustrate rotor slot embodiments of the present inventionthat are useful for creating inductance modulation in induction motorsdesigned for sensorless control.

In one embodiment of the present invention, as shown in FIG. 1, a rotor10 with a shaft 16 and a center point 18 of rotation has a plurality ofsaturable slot bridges 13 which are formed between rotor slots 12 androtor slit patterns which are shown in FIG. 1 as air slits 14. Asdiscussed below, a rotor slit pattern may comprise one or more air slitsor slits which include rotor cage material.

The rotor slots and air slit combinations of FIG. 1 are suitable forusing rotor cage material 85 which comprises either fabricated or castrotor cages. Typically fabricated rotor cages include copper rotor bars,and cast rotor cages are made with molten aluminum. The rotor slotbridges enable the rotor to be cast without requiring special castingjigs or containment fixtures to prevent molten aluminum from escapingthe rotor slots, while still introducing a desired spatial modulation inrotor leakage inductance.

The desired spatial modulation in rotor leakage inductance forsensorless control can be created by varying the length of the airslits. In one embodiment, the distances of the rotor slots from thecenter point 18 of the rotor lamination are additionally varied. FIG. 1shows these variations for a two pole machine where the air slit lengthdecreases from air slit 40 through air slits 42, 44, 46, 48, 50, and 52and then increases.

Rotor slots positioned with shallow air slits (with the shallowestrepresented by air slit 52) result in a lower localized leakageinductance compared to slots with deep air slits. A modulation that istrackable from the stator windings is obtained by having the variationin length periodic with each pole, as shown. In FIG. 1, uniform widthsof the air slits are used to maintain a uniform and symmetricmagnetizing inductance and thereby avoid the introduction of undesirablesynchronous reluctance torque.

The slots are designed to maximize the spatial modulation in rotorleakage inductance created by variation in the air slits whileminimizing the variation caused by the rotor slot bridge saturationunder loaded operating conditions. To accomplish this, the bridge 13height (ah) should be as short as possible, and the air slits should beas narrow and have an average height (sh) as long as possible tomaximize the desired saliency. Limitations on narrowness and averagelength will be set primarily by manufacturing considerations, althoughundesirable saturation of the slit region can occur if it is too narrowunder loaded operating conditions. Thinner laminations generally enablenarrower slot structures to be punched.

The laminations are stacked such that slots of equal air slit length arealigned axially. As is common practice, the laminations can be rotatedduring stacking to prevent uneven stack build-up due to non-uniformlamination thickness. The only restriction is that the rotations for atwo pole embodiment are in increments of one pole pitch (180 electricaldegrees). No restrictions are placed on the common practice of rotorskewing.

Rotor laminations 10 can be punched by compound dies or by indexednotching dies. Compound dies include dies such as progressive dies andcluster dies which can punch the rotor slots (and slits if applicable)with a single punching step. These dies are expensive to create and aregood for high volume applications. Indexed notching dies typicallycreate a single slot and/or slit at a time with the die or the rotorlamination being rotated prior to the punching of the next slot.

FIG. 2 is a top view of rotor lamination positions during indexed diepunching according to one embodiment of the present invention. Anindexed die is not shown in FIG. 2, but one example of an indexed die isprovided in FIG. 8. A die for forming slits in FIG. 2 would have oneslot punch and one slit punch.

In the embodiment of FIG. 2, a single notching die can be used byvarying the position of the rotor lamination center point relative tothe notching die. This can be achieved by either moving the notching dieas the rotor lamination is indexed or, preferably, by moving thelocation of the rotor lamination center point as it is indexed whilekeeping the die fixed. As shown in FIG. 2, rotor lamination center point118 can be shifted by a distance d from rotor lamination center point 18to create a slot 112/air slit 114 combination 22 which has an air slitwhich is shorter by distance d than the slot 12/air slit 14 combination20. Existing indexed punch presses can be modified via the addition of aservo mechanism to provide the controlled movement of the rotorlamination center point.

FIG. 3 is top view of a four pole rotor lamination of the presentinvention, In this embodiment, a combination of open rotor slots 77(shown as open in FIG. 5 and in a pre-opened form in FIG. 3) and closedrotor slots 76 with air slits 74 (separated by bridges 73) are used tocreate the desired variation in rotor leakage inductance. The rotorslots are all initially formed as closed rotor slots as shown in FIG. 3to facilitate casting. A progressive die can be used or single slot diecan be used to create the variation via translation of the indexingmachine spindle as per FIG. 2. Key ways (not shown) can be used to helpwith alignment.

FIG. 4 is a top view of a masking lamination 800 having openings 810which can be used during rotor cage casting to prevent molten materialfrom entering the air slits. This masking lamination can be placed ateach end of the rotor stack aligned with the slots of lamination 10prior to casting. The masking laminations can be created in a similarmanner as lamination 10 via a cluster or single indexed notching diewhich does not have the air slit features.

FIG. 5 is a top view of the lamination of FIG. 3 after casting rotorcage material 85 and machining the outer surface 9 of the rotorlamination to obtain the desired diameter. After molten material 85 ispoured in the rotor slots and is hardened, open slots 77 can be createdby a machining operation such as for example, by turning the laminationstack on a lathe to remove the slot bridge on specific slots. The openslots are less sensitive to saturation effects than closed slots 84 andthus should enable improved controllability.

FIG. 6 is a top view of another rotor lamination 10 of the presentinvention. In this embodiment, rotor slots 12 are each at about the samedistance from the center point 18, and the rotor leakage inductancemodulation is created by using air slits with varying width. As shown inthe two pole configuration of FIG. 6, some of air slits 210 havenarrower widths than other air slits 212. A wider slit will produce asmaller rotor leakage inductance than a narrower slit.

Each slot has two effective rotor slot bridges. An inner bridge 13 isclosest to the rotor conductor 85 and provides a barrier to retain themolten aluminum during casting. An optional outer bridge 15 facilitatesthe turning (machining) of the rotor outer diameter and minimizeslamination smearing. (A similar outer bridge can be added to the airslit 74 in the FIG. 3 embodiment if a compound type die is used forpunching.) The bridges should be designed to be as thin as possible tominimize the effects of saturation on the overall rotor leakageinductance. Ideally, the inductance associated with the air slit regionshould dominate the overall rotor leakage inductance.

Either compound or indexed notching dies can used to punch the rotorlaminations. With indexed notching dies, as commonly used for largermotors, the manufacturing of the rotor lamination can be accomplished ineither two or three steps.

In one embodiment, as shown in FIG. 7, a symmetrical lamination is firstcreated with narrow slits 214. This lamination can be fabricated usingeither one or two passes through a punching press. For a one passfabrication technique, the notching die must include both the rotor slotpunch and the air slit punch. Alternatively, separate dies can be usedto create the rotor slots and the air slits using two separate passes. Afinal pass using a wider air slit die is then used to introduce thespatial modulation by punching over selected air slits for each pole.This embodiment uses a punch press having the capability of indexing therotor lamination without punching.

FIG. 8 is a perspective view of a variable depth notching die 216 of thepresent invention which can be used to fabricate the rotor slots 12 androtor slit patterns 237 including air slits 236, 238, and 240 of FIGS. 9and 10 in a single pass. Depending on the depth of closure of the die,rotor slot punching 222 and one or both of the two rotor slit punchings226 and 228 can punch through the rotor lamination 10. In FIG. 9, thedie was closed far enough to create two air slits 238 and 236. In FIG.10, the die was closed far enough for punching 228 to form air slit 240but not far enough for the shorter punching 226 to form an air slit.

The advantage of this die is that it enables the rotor lamination to bepunched in a single pass through the notching punch press. The pressmust have the capability of varying between two stroke lengths at eachindexed position. The increased complexity of the die is a limitation.The two air slit punching portions of the die should be designed to bereplaceable since they may be prone to breakage.

FIG. 11 is a top view of another rotor lamination of the presentinvention showing a two pole design formed by the die of FIG. 8. Thisembodiment can alternatively be formed using separate dies over severalpasses. In one embodiment, one die can be used for the rotor conductorslot area in the first pass and a variable depth die for the two airslits in the second pass. In another embodiment, three separate dies andpasses can be used.

FIG. 12 is a top view of a four pole rotor lamination 10 of the presentinvention wherein the air slits 14 do not extend through the outersurface of rotor lamination 10. As in FIG. 1, the length of the airslits decreases from air slit 60 through air slits 62, 64 and 66. Unlikethe embodiment of FIG. 1, in the embodiment of FIG. 12, the rotor slotsare open to the rotor slits, and the resulting difficulty in preventingmolten aluminum from flowing in the air slits generally precludes theuse with cast cage rotors. Furthermore, the spatial modulation cannot becreated by a single indexed notching die. For this embodiment, acompound or progressive die is used. One advantage to the embodiment ofFIG. 12 is that machining of the outer surface of a rotor is simplerthan machining of the surface of a rotor formed with laminations of FIG.1 wherein the slits are open to the outer rotor surface.

FIG. 13-16 are top views illustrating another fabrication technique forrotor laminations. In this embodiment, an asymmetrical rotor slot/slitshape combination is created and used with slit length variation andperiodic lamination flipping and rotation to create the desired rotorleakage inductance modulation with low rotor leakage saturation effectsand low harmonic loss.

First, a rotor slot die is used to create slits 242 in rotor lamination10 of FIG. 13. In one embodiment, the slits are not air slits aspreviously discussed, but will instead be filled with rotor cage castingmaterial. In this embodiment, bridges 241 between the slits and theouter surface of the lamination are preferably present to retain moltenaluminum during rotor cage casting. The thickness of the bridge ispreferably such that it is entirely removable during a machiningoperation on a lathe to create the desired rotor outside diameter. Aswith the previous embodiments, the width of the slit is chosen tominimize saturation of the tooth tip and slit region during loadingwhile still creating the desired net rotor leakage inductance. The width(sw) will typically vary from about 0.152 centimeters (0.060 inches) toabout 0.305 centimeters (0.120 inches) for industrial size motors(1-1000 hp) of cast cage construction.

Next, a rotor slot die shape is used to create slots 246 and 248 in thesixty-six slot, six pole rotor lamination of FIG. 14. A variation in thelength of the rotor slit can be created by translating the laminationcenter point of the die as the lamination is indexed. The variation inlength results in a spatial modulation in the rotor leakage inductance.In addition to the length variation, the slots are punched off centerwith respect to slits 242. Key way 78 and additional key ways 244 can bepunched to facilitate lamination orientation. The lamination of FIG. 14can alternatively be created using a progressive die.

FIG. 15 illustrates a lamination of FIG. 14 after it has been flippedand rotated. During the creation of rotor stacks (shown as stacks 300 inFIGS. 17 and 18), selected laminations or groups of laminations arealternately flipped and rotated as shown in FIG. 15 prior to stacking.

After stacking, the rotor cage can be cast and the rotor can be machinedto the desired diameter. Unlike the above embodiments, the cast aluminumis allowed to completely fill the slit region. During the machining, therotor slot bridges are preferably removed to create open slots andexpose the cage aluminum as shown in FIG. 16.

FIGS. 17 and 18 are sectional side views of selected lamination stackingembodiments. In FIG. 17, every fifth lamination is flipped horizontallyand rotated by six slots counterclockwise. In FIG. 18, the laminationsare flipped and rotated in groups of four. Because the rotor slots areasymmetrical, the electrical path through the slit region is effectivelydiscontinuous even though it is filled with cast aluminum; i.e., theslit region appears to be nonconductive to all but very high frequencycurrents. Electrical currents including the AC signal currents used forsensorless control are therefore largely excluded from the slit regionand confined predominately to the main rotor slot region. With the ACsignal currents confined to the main slot region, a variation in leakageinductance at the AC signal frequency is created that is directlydependent upon the variation in length of the slits.

The optimum number of laminations per grouping between flipping androtation depends on the lamination thickness as well as the frequenciesof the inverter harmonics and the signal injection. The total distanceof each grouping preferably ranges from about 1/8 of an inch (0.32centimeters) to about 1/2 of an inch (1.27 centimeters). Thickerlaminations stack to a total distance with fewer laminations thanthinner laminations stacking to the same distance. Higher frequenciesrequire more frequent lamination flipping. To minimize harmonic lossescaused by inverter excitation, the lamination flipping should generallybe as frequent as is practical. Too much flipping will increase theamount of undesirable flux that jumps crosswise from one lamination edgeto another in the slit region. To achieve the proper laminationorientation during stacking, the amount of rotation will depend upon thenumber of slots, the number of poles, and the slit depth variationpattern. During stacking, the laminations can be successively rotated byone or more pole pitches to maintain stack uniformity.

Removal of slot bridges 241 during machining significantly reduces theunwanted effects of localized tooth tip and slot bridge saturation dueto loading. The fundamental current flowing at slip frequency is alsoconfined to the main rotor slot region and therefore experiences auniform rotor bar resistance. Thus torque production is not degraded bythe length modulation.

FIGS. 19-32 illustrate rotor slot embodiments (and several associatedgraphs) of the present invention that are useful for creating resistancemodulation in induction motors designed for sensorless control. Theseembodiments are particularly useful with cast rotors in closed rotorslot embodiments. The majority of induction motors with cast rotor cageshave closed rotor slots to minimize manufacturing costs. However, theslot designs can also be used in open or semi-closed rotor slot designsto achieve the same desirable rotor resistance asymmetry or spatialmodulation.

FIG. 19 is a top view of two rotor slots 412 and 414 of a rotor 410 ofthe present invention with rotor slot 414 including an extension 416.Preferably, the rotor slot shapes are of substantially equalcross-sectional areas such that the conductive bars 413 and 415 of theslots, including the bar extensions, have substantially equalresistances at direct current and low slip frequencies, particularly atrated slip frequency of the motor. Substantially equal resistances atrated slip frequency are important to ensuring that undesirable torquepulsations are not introduced at slip frequency.

Typically rated slip frequencies are in the range of 0.1 Hz to 5 HZdepending upon the particular motor size and design and the inverterswitching frequency. Typical frequencies at which an AC signal will beinjected for resistance modulation range from about 20 to about 2000 Hz.

As rotor excitation frequencies increase past slip frequency, rotor barresistances increase. The rate of increasing rotor bar resistance ishigher for the shape of slot 414 than for the shape of slot 412 due toconductor skin effects. As rotor excitation frequencies increase pastthe AC signal injection frequency, the difference in the increasesbecomes more substantial. The skin effects force the high slip frequencycurrent to the outer portion of rotor bar into the rotor extension 416.Although shown as rectangular, rotor extension 416 may comprise anyshape capable of creating the desired resistance profile. Other usefulshapes, for example, include semicircles and ovals.

FIG. 20 is a top view of the rotor slots of FIG. 19 in a lamination foruse in a two pole machine. By using the two slot shapes 412 and 414 inthis manner, a desired spatial modulation in the net rotor resistance asseen at the injection frequency from stator terminals (not shown) can becreated. With a sinusoidally distributed stator winding, the effectiverotor resistance can achieve a nearly sinusoidal spatial modulation eventhough the individual bar resistances change abruptly.

FIG. 21 is a top view of rotor slots in a lamination for use in a fourpole machine illustrating an embodiment where the individual barresistances change less abruptly than the resistances of FIG. 20. InFIG. 21, the heights (h) of the extensions of the rotor bars areincrementally increased with extension 416 being shorter than extension418 which in turn is shorter than extension 420.

The embodiments of FIGS. 19-21 are capable of achieving trackablespatial modulation in rotor resistance. The extensions do createadditional harmonic losses with inverter switching and thus can reducemotor efficiency and create increased motor heat loss.

FIGS. 22-23, 25-30, and 32 are top views of additional rotor slot shapevariations of the present invention which do not require extensions.These slot shapes can be replicated around a rotor lamination in amanner similar to that shown in FIG. 20 or 21, for example.

In FIG. 22, some of the of rotor slots 412 have outer slot surfaces 424facing an outer perimeter 423 of the laminations that are longer thanouter slot surfaces 426 of some others of the plurality of rotor slots422 that face the outer perimeter of the laminations while the crosssectional areas of the rotor slots remain substantially equal. As statedabove, preferably, the cross sectional areas are within one percent ofeach other such that the DC resistances of each bar are essentiallyidentical. Although the differences in bar resistances at higher rotorexcitations frequencies is not as dramatic as the embodiment of FIG. 19,a sufficiently large rotor resistance modulation can still be obtainedat a signal injection frequency without excessive harmonic losses at theinverter switching frequencies.

In FIG. 23, the harmonic losses at the inverter switching frequenciesare further reduced narrowing the rotor slot 430 at portions 434. FIG.24 is a simulation graph of rotor bar resistances as a function of rotorexcitation for the embodiment of FIG. 23. By narrowing the slot justbelow outer slot surface 432 while maintaining substantially equaldistances for outer slot surfaces 424 and 432, and substantially equalcross-sectional areas, the resistances of the rotor bars aresubstantially equal at slip frequencies and inverter switching harmonicswhile the resistance modulation is maintained in the area of the signalinjection frequency. Thus no significant increase in losses are createdat the inverter switching harmonics.

The present invention includes other slot/bar shapes that achieve rotorresistance modulation for tracking rotor position and velocity. Forexample, in FIG. 25, the distances of outer slot surfaces 422 and 432and about the same, but, by narrowing slot side surfaces 434 just belowouter slot surface 432 and maintaining the slot 436 at side surfaces 438just below outer slot surface 433, rotor resistance modulation iscreated at signal injection frequencies.

In FIG. 26, air slits 440 are added between slots 412 and 430 and theouter perimeter 441 of the rotor to minimize variation in rotor leakageinductance due to saturation. In FIG. 27, the widths of air slits 440and 442 are different to create a spatial modulation in rotor leakageinductance in addition to, and aligned with, the modulation of rotorresistance.

In FIG. 28, slot 444 includes a bridge 448 with slit 456 extendingtherein to further minimize bar resistance of bar 445 at signalinjection and inverter switching frequencies.

In FIG. 29, both slots 444 and 446 have bridges 448 and 450 permittingair slits 456 and 458 to extend into the main slot portion to minimizeresistance at signal injection and inverter switching frequencies withresistance modulation at signal injection frequency for tracking beingcreated by differing slot/bar distances just below the outer surfaces ofthe slots/bars.

In FIG. 30, slots 470 and 472 each have similar outer slot surfaces 474and 476 with respective first side surfaces 478 and 480 having differentangles and second side surfaces 479 and 481 having different angles withrespect to respective inner slot surfaces 475 and 477. FIG. 31 is asimulation graph of rotor bar resistances as a function of rotorexcitation for the embodiment of FIG. 30. In this embodiment theresistances of the rotor bars are substantially equal at slipfrequencies, the resistance modulation is maintained in the area of thesignal injection frequency, and some losses are created at the inverterswitching harmonics as the resistance of a rotor bar in slot 470increases at a faster rate than the resistance of a rotor bar in slot472. In another related embodiment (not shown), outer slot surfaces 474and 476 have different lengths. In this embodiment the cross sectionalareas can remain constant even if the angles of the side surfaces arenot different.

FIG. 32 is a top view of an additional rotor slot shape variation of thepresent invention. In FIG. 32, slots 482 and 484 have respective curvedouter slot surfaces 486 and 488 with different radii of curvature. Slots482 and 484 can optionally have respective side surfaces 490 and 492with different angles with respect to inner slot surfaces 494 and 496.This embodiment provides increased resistance in the rotor bar of slot482 in the area of the signal injection frequency.

After punching and stacking laminations formed with the slots describedwith respect to FIGS. 19-32, conventional rotor cage die casting methodscan be used to fabricate the rotor. A key-way or one or more alignmentholes is useful to ensure correct orientation of the rotor laminationsduring punching and stacking. If compound dies are used tosimultaneously punch all rotor slots in a lamination, no specialfixtures are needed.

If indexed notching dies are used to punch rotor slots, multiple dieswill be used with one die being used for each shape, and the punch pressshould be capable of indexing the rotor lamination without punching.

FIGS. 33-38 illustrate rotor slot embodiments of the present inventionthat are useful for creating inductance (FIGS. 33-38) and resistance(FIGS. 36-38) modulation in induction motors designed for sensorlesscontrol. In these embodiments the slot shapes are uniform and themodulation is achieved by rotor lamination shape or by slot inserts.

FIG. 33 is a top view of a rotor lamination 510 of the present inventionhaving an oblong shape. In this embodiment, slots 516 are substantiallyidentical and each spaced about the same distance from a center point511 of rotation. The blanking die (not shown) for the rotor laminationshowever is shaped to create the desired modulation. Dotted line 512illustrates a normal round (circular) rotor shape whereas solid line 514illustrates the shape of the rotor of this embodiment of the presentinvention.

The spatial modulation in inductance results because the oblong shape ofthe lamination causes slot 515 to be closer to the outer edge 514 thanslot 516. Rotor slot 515 will thus have a lower magnetizing and leakageinductance than slot 516. The embodiment of FIG. 33 is appropriate for atwo pole machine. For a four pole machine, an additional pair ofelongated portions (not shown) could be formed around slots 513 and 515,for example.

FIG. 34 is a top view of a rotor lamination of the present inventionillustrating the insertion of magnetic inserts 522, 524, and 526 betweenrotor bars 520 and the outer perimeter 523 of the rotor laminations.FIG. 34 also illustrates the use of open rotor slots 518. Closed rotorslot embodiments are shown in FIGS. 37 and 38.

In one embodiment, the magnetic inserts comprise a molded magneticmaterial. One example of a useful molded material for applications wheretemperatures do not exceed 200° C. is an iron powder mixed with aplastic or an epoxy. For higher temperatures, iron flakes can becompacted without a binder.

The slots are shaped to accommodate the insertion of various shapesand/or numbers of magnetic inserts. The magnetic inserts can either beinserted before or after the insertion of rotor bars depending on theparticular design.

If the rotor bars are prefabricated, it is convenient to position themagnetic inserts after inserting the rotor bars. If the rotor bars willbe cast in the rotor slots, then at least one magnetic insert will bepositioned in each slot prior to rotor bar casting. For cast rotor bars,it is useful to coat the magnetic inserts with an inorganic material,shown for example as inorganic coating 521, to fill any pores of themagnetic inserts and increase the mechanical strength of the magneticinserts.

In the example of FIG. 34, slot 518 has no magnetic insert, slot 530 hasone magnetic insert, and slot 532 has two magnetic inserts to create arelatively smooth variation in leakage inductance.

Generally the maximum achievable relative permeability of moldedmagnetic materials is only about 350 to 400. Lower permeabilities can beused to reduce rotor saturation and flux. These lower permeabilities areeasily achieved by reduced molding pressure and/or by mixingnon-magnetic materials with the magnetic materials prior to molding.Thus, inductance modulation may be accomplished by inserting multiplelayers or sizes of materials of like permeability or by making pieces ofdifferent permeabilities. In either embodiment, the amount of addedinductance can be regulated by the molded density as well as thegeometry.

FIG. 35 is a view of a magnetic slot wedge formed with a magnetic insertcomprising laminations 550 surrounding a rod 552. In one embodiment thelaminations comprise iron or steel and the bar comprises a glasscomposite rod. This embodiment is useful for prefabricated rotor bars.Preferably slot 544 includes notches 556 for insertion of thelaminations.

FIGS. 36-38 are additional views of magnetic slot wedges of the presentinvention which are useful for modulation in embodiments which do notrequire multiple notching dies. In these embodiments, rotor slots of alamination 610 can be formed with a single slot shape. Differentmagnetic insert/rotor bar 614 shapes can then be used to providedifferent resistance modulation patterns for the uniform slot shapes.Because permeability of a magnetic slot wedge can be adjusted over awide range, such adjustments can be used to enhance the inductancevariation in addition to (to enhance sensitivity or reliability) orinstead of the resistance variation.

In FIGS. 36 and 37, for example, magnetic inserts 616 can be adjusted sothat the width w and/or height h varies and the correspondingprojections 615 of rotor bars 614 thus vary. These embodiments areparticularly useful for cast rotor bars. In FIG. 36, the slots 612 areopen and notches 613 are used to assist in holding the magnetic inserts615 in position. In FIG. 37, the slots 630 are closed.

For prefabricated rotor bars, it may be impractical to have differentshaped rotor bars. In the embodiment of FIG. 38, rotor bar main portions652 can each have the same shape with resistance modulation provided byusing filler bars 656 having different heights h and/or widths w for theextensions. The magnetic inserts 654 and filler bars 656 can efficientlybe made in a variety of shapes and/or sizes resulting in reduced costsand enhanced flexibility. The magnetic insert may be formed around thefiller bar after the filler bar and rotor bar are positioned, or themagnetic insert may be positioned around the filler bar prior toinsertion into the slot.

Another method for achieving resistance modulation is to use theembodiment of FIG. 35 but to vary the size of rod 552 and change thematerial of rod 52 to copper or aluminum.

While only certain preferred features of the invention have beenillustrated and described herein, many modifications and changes willoccur to those skilled in the art. It is, therefore, to be understoodthat the appended claims are intended to cover all such modificationsand changes as fall within the true spirit of the invention.

We claim:
 1. A method for fabricating an induction motor rotorcomprises:stacking a plurality of rotor laminations, each of the rotorlaminations including a plurality of closed rotor slots havingsubstantially similar shapes, some of the closed rotor slots beingpositioned farther from a center point of rotation than the others ofthe closed rotor slots; filling the rotor slots with rotor cagematerial; machining an outer surface of the stacked rotor laminations soas to cause the some of the rotor slots to become open rotor slots.
 2. Amethod for fabricating an induction motor rotor comprises:stacking aplurality of rotor laminations, each of the rotor laminations includinga plurality of closed rotor slots, some of the closed rotor slots beingpositioned farther from a center point of rotation than the others ofthe closed rotor slots, and a plurality of rotor slits, each of therotor slits being situated between a respective one of the others of theclosed rotor slots and the outer surface of the rotor lamination fillingthe rotor slots with rotor cage material; machining an outer surface ofthe stacked rotor laminations so as to cause the some of the rotor slotsto become open rotor slots.
 3. The method of claim 2 wherein each of therotor slits is separated from its respective rotor slot by a bridge. 4.The method of claim 3 further including, prior to filling the slots withrotor cage material, positioning a masking lamination over the stackedlaminations to prevent the rotor cage material from entering the rotorslits.
 5. The method of claim 3 wherein the machining is performed so asto ensure that each rotor slit is open to the outer surface of the rotorlamination.
 6. A method for fabricating an induction motor rotorcomprises:providing a plurality of rotor laminations each including aplurality of rotor slots situated at different distances from a centerpoint of rotation and a plurality of rotor slits, each rotor slit beingopen to and off center with respect to its respective rotor slot;stacking the plurality of rotor laminations by positioning some of therotor laminations in an orientation which is turned over and rotatedwith respect to others of the rotor laminations; filling the rotor slotsand the rotor slits with cage material; machining outer surfaces of therotor laminations.
 7. The method of claim 6 wherein the machining isperformed at least until each rotor slit is open to an outer surface ofthe respective rotor lamination.